Optical methods to determine the H+-concentration, e.g. expressed as pH, have been known for many decades. Typically, use is made of a dye of which an optical property (such as extinction at a certain wavelength or fluorescence) changes as it reacts to H+ present in a sample. Thus, the pH of the sample can be indicated. For this reason such dyes are also referred to as pH-indicators or H+ indicators, or in short indicators. Various examples of suitable dyes, together with the pH ranges within which they change colour or another optical property, for instance fluorescence, are generally known in the art, and described in various handbooks, e.g. in “Indicators”, E. Bishop, Pergamon Press, 1972, Chapter 3. or ‘Haugland, Molecular Probes® Handbook—A Guide to Fluorescent Probes and Labelling Technologies, 11th Ed. (2010), e.g. Table 20.1, 20.3 (by Molecular Probes, Inc., Eugene, Oreg., USA) The dye may be added to a liquid of which the pH is to be determined or may be immobilised on a support. In many application the dyes change colour when contacted with a liquid comprising H+, and the colour change can be detected with the naked eye. Alternatively, use can be made of an optical detection system.
U.S. Pat. No. 5,853,669 relates to a sensor membrane for the reversible optical indication of the pH of a sample, having a mechanically stable support element and an indicator dye layer disposed thereon, which comprises a hydrophilic homogeneous, non-fibrous accommodating layer disposed on at least one side of the support element, which layer contains the indicator dye in an immobilized form
WO 01/13097 relates to a pH sensor for the visual or optical indication of the pH of a sample, the pH sensor comprising a hydrophilic, intrinsically charged or neutral, synthetic membrane and at least one pH indicator dye immobilized thereto, so as to prevent appreciated bleeding of the at least one indicator from the synthetic membrane upon immersion in an aqueous liquid.
The measurement of the H+-concentration in a sample can be disturbed by various factors, including the potential presence of other dyes in a sample, reactivity of the indicator towards other components in a sample, lipids and salts. This may adversely affect the measurement to a large extent, in particular in case the product of which the H+ concentration is to be determined has a complex composition, as is, e.g. the case for food products for humans, feeds for animals and nutrient products for plants.